Conventionally, when coloring an object such as paper, a plastic, cloth, a can, and a bottle, there has generally been employed a method in which an ink, Japanese ink, a coating, or the like each having a color is applied to the object and adheres thereto by the evaporation of a solvent to obtain a colored material. However, in recent years, as a result of the need for a system which can immediately respond to any character, view, and pattern, a heat-sensitive recording material in which a layer having color-developing ability (hereinafter, referred to as a recording layer) is previously applied to the whole surface has been developed. In the system using this recording material, an electric signal is converted into heat by a thermal head, and various characters and patterns are immediately revealed and recorded. Since a vivid developed color image is easily obtained with a simple apparatus, this recording material has been widely put to practical use as a facsimile paper, a word processor paper, a chart of various analytical instruments, a ticket, a commuter pass, a prepaid card, a tag, and the like.
Generally, a system using an electron-donative dye (a phthalide or a fluoran leuco dye) as a color coupler and an electron-acceptive phenol compound as a color developer is well known as a recording layer of the heat-sensitive recording material (Patent Literature 1). However, since a reversible reaction is used in this system, the natural discoloration of a developed color image and the decolorization by a plasticizer, an oil, water, heat, light, and the like may easily occur, and, therefore, it has a problem that the storage stability of the developed color image is poor.
In order to overcome this disadvantage, various compounds have been studied, and an attempt utilizing a phthalocyanine compound having high fastness has also been proposed. This is a method of containing a precursor of a phthalocyanine compound in a recording layer and heating the precursor to form a phthalocyanine compound as a developed color image. For example, there has been proposed a method of using 1-amino-3-imino-isoindolenine or a derivative thereof and an organic acid metal salt or metal complex salt compound for a recording layer (Patent Literature 2). Furthermore, there has been proposed a heat-sensitive recording material in which an imino compound such as 1,3-diiminoisoindoline is carried (Patent Literature 3). However, in any of the cases, the hue of the developed color images was not vivid, and it was difficult to actually use these materials as a heat-sensitive recording material.
Furthermore, there has also been proposed methods of using a heat-sensitive recording material in which an irreversible developed color body obtained by a heat-melting reaction of an aromatic isocyanate with an imino compound is used as a recording image (Patent Literatures 4 and 5), but these methods had a disadvantage that the storage stability of the developed color image was poor.
For such reasons, there has been required a heat-sensitive recording material which solves the conventional problems and satisfies high fastness and a vivid hue at the same time.